As shown in JP-U-4-105906A, the VTC changes valve timing of an intake valve and/or an exhaust valve by rotational torque of an electric motor. A driving circuit receives a control signal from a control circuit and controls the motor based on the control signal. While the valve timing is maintained constant, a rotational phase of the motor must be constant relative to the crankshaft. When the rotational phase of the motor relative to the crankshaft is varied, a rotational phase of the camshaft relative to the crankshaft is varied whereby the valve timing is varied. In order to maintain the rotational phase of the motor relative to the crankshaft, the current supplied to the motor in controlled. The control circuit generates a control voltage signal which is in proportion to a target rotation speed of the motor, and the driving circuit controls the motor in such a manner that an actual rotation speed of the motor coincides with a target rotation speed represented by the control voltage signal.
In the VTC mounted on a vehicle, because a voltage of the control voltage signal has an upper limit, a resolution of the target rotation speed has also an upper limit. Thus, the rotation of the motor cannot follow the rotation of the crankshaft, which frequently changes according to the driving condition of the engine. A rotational phase may be changed unintentionally.